BOSTON — The Federal Aviation Administration is warning pilots that it has created a no-fly zone over the site of two explosions at the annual Boston marathon.

The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston Street. The zone is limited to flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing.

The notice says the no-fly zone is effective immediately, and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.

“We haven’t had any impact on Portland operations at this time,” said Paul Bradbury, manager of the Portland Jetport. “There’s been no further notice of the threat or to have any reason the threat extend this far north.”